1. Field of the Invention
The present invention relates generally to a thyristor and specifically to a light-triggerable power thyristor.
A thyristor already ignites before the static trigger voltage U.sub.BO is reached when the time change dU/dt of the voltage U applied between its terminals exceeds a specific maximum value for the respective thyristor. This misignition that may lead to the destruction of the thyristor under certain circumstances is caused by the build-up of the space-charge zone at the p-base/n-base junction and the shift current I.sub.d =C.sub.d dU/dt (C.sub.d : voltage-dependent space charge capacitance of the pn-junction) resulting therefrom that intensifies the blocking current. The dU/dt strength of a thyristor can be substantially improved with the assistance of cathode-emitter shorts, since a majority of the capacitative shift current I.sub.d in the semiconductor body no longer flows over the pn-junction of the cathode side but directly from the base to the cathode. The critical value of the voltage steepness of correspondingly constructed thyristors typically amounts to several 1000 V/.mu.s.
2. Description of Related Art
Power thyristors must be provided with emitter shorts in order to meet the demands with respect to the dU/dt loadability. Emitter shorts, however, divert a part of the gate current directly to the cathode, which has a disadvantageous effect on the trigger sensitivity of the thyristor. Among the things proposed for solving this problem is to optimize the structure of the gate, to compensate the change in potential of the base of the cathode side caused by the capacitative current I.sub.d and to increase the optical trigger sensitivity of the thyristor with a plurality of "amplifying gate" stages connected following one another (see, for example, the reference to Paul D. Taylor, "Thyristor Design and Realization", John Wiley & Sons (1987), pp. 133-141, and the reference IEEE Transactions on Electron Devices, Vol. ED-30, No. 7, July 1983, pp. 816-824.